The use of thermal energy to destroy bodily tissue can be applied to a variety of therapeutic procedures, including the destruction of tumors. Thermal energy can be imparted to the tissue using various forms of energy, such as radio frequency electrical energy, microwave or light wave electromagnetic energy, or ultrasonic vibrational energy. Radio frequency (RF) ablation, for example, can be effected by placing one or more electrodes against or into tissue to be treated and passing high frequency electrical current into the tissue. The current can flow between closely spaced emitting electrodes or between an emitting electrode and a larger, common electrode located remotely from the tissue to be heated.
One disadvantage with these techniques is that maximum heating often occurs at or near the interface between the therapeutic tool and the tissue. In RF ablation, for example, maximum heating can occur in the tissue immediately adjacent to the emitting electrode. This can reduce the conductivity of the tissue, and in some cases, can cause water within the tissue to boil and become water vapor. As this process continues, the impedance of the tissue can increase and prevent current from entering into the surrounding tissue. Thus, conventional RF instruments are limited in the volume of tissue that can be treated.
Fluid enhanced ablation therapy, such as the SERF™ ablation technique (Saline Enhanced Radio Frequency™ ablation), can treat a greater volume of tissue than conventional RF ablation. The SERF ablation technique is described in U.S. Pat. No. 6,328,735, which is hereby incorporated by reference in its entirety. Using the SERF ablation technique, saline is passed through a needle and heated, and the heated fluid is delivered to the tissue immediately surrounding the needle. The saline helps distribute the heat developed adjacent to the needle and thereby allows a greater volume of tissue to be treated with a therapeutic dose of ablative energy. The therapy is usually completed once a target volume of tissue reaches a desired therapeutic temperature, or otherwise receives a therapeutic dose of energy.
One problem that can arise in fluid enhanced ablation therapy is that gas dissolved in the fluid can come out of solution due to heating that occurs before or during its introduction into the volume of tissue to be treated. When gas comes out of solution, it introduces a compressible gas into a system otherwise filled with an incompressible fluid. The compliance of the compressible gas can introduce a number of complications into the fluid enhanced ablation system and, as the amount of compliance in the system increases, the efficiency and effectiveness of the treatment can decrease. For example, bubbles formed from gas coming out of solution in the fluid (e.g., as the result of localized super-heating of the fluid near an RF electrode) can affect the fluid flow rate since the gas bubbles are compressible and can absorb pressure created by a fluid pump. Variance in the fluid flow rate can, in turn, reduce the volume of tissue that can be treated and make ablation therapy less reliable and reproducible. Still further, introducing gas bubbles into tissue within the body can, in some circumstances, have unintended and undesirable medical consequences for a patient.
Accordingly, there remains a need for improved devices and methods for fluid enhanced ablation therapy.